Image forming apparatus

Cross-reference to related application

This application claims priority to Japanese Patent Application No. 2010-070865, which was filed on Mar. 25, 2010, the contents of which are incorporated herein by reference in its entirety.

Background of the invention

1. Field of the invention

The present invention relates to an image forming apparatus.

2. Description of the related art

An electrophotographic image forming apparatus widely employed in a copying machine, a laser printer, a facsimile and the like is equipped with a fixing device of a heat fixing system. The fixing device of a heat fixing system comprises a fixing roller and a pressure roller. The fixing roller and the pressure roller each comprise a core metal comprising a metal or the like, and an elastomer layer comprising a resin or a rubber, having excellent releasability, formed on a surface of the core metal. A heater as a heat source is provided inside the fixing roller, and the heater heats the fixing roll and the pressure roller so as to reach a given temperature by application of heat generated by the heater. In the fixing device of a heat fixing system, the fixing roller and the pressure roller are heated to a given temperature, and the heated fixing roller and pressure roller are then press-contacted with each other. As a result, when a recording medium having an unfixed toner image borne thereon passes through a fixing nip region formed between those rollers, those rollers fuse a toner constituting the unfixed toner image by application of heat, thereby fixing the unfixed toner image to the recording medium. The fixing device of a heat fixing system holds the whole of the fixing roller and the pressure roller at a given temperature, and is therefore suitable for high speed printing.

However, the fixing device of a heat fixing system is required to heat the fixing roller and the pressure roller to a given temperature by a heater provided inside the fixing roller at the time of power activation of an image forming apparatus, and this relatively increases the time of from the power activation to a printable state (warming-up time). For this reason, there is the problem that the time of from power activation by a user to output of a recording medium having an image printed thereon becomes long.

A method for shortening a warming-up time includes a method of decreasing heat capacity of a fixing roller by, for example, decreasing a thickness of a core metal of the fixing roller. A method of shortening the time until a recording medium having an image printed thereon is outputted includes a method of initiating a paper-passing operation at a fraction of the time after reaching a surface temperature of the fixing roller to a given fixable temperature.

However, the fixing device equipped with a fixing roller having small heat capacity involves the case that storage of heat of the fixing roller is not sufficient just after completion of the warming-up. In such a case, heat on the surface of the fixing roller is absorbed by the recording medium passing through the fixing nip region at the time of fixing operation, and the surface temperature of the fixing roller cannot be maintained at a given fixable temperature, leading to occurrence of poor fixing.

To achieve energy saving by reducing consumed power as much as possible, the time until moving from a standby mode (ready state) that performs heat retention of the fixing roller during stopping job to a sleep mode that does not perform heat retention of the fixing roller tends to be shortened. For this reason, in some cases, storage of heat of the fixing roller may be not sufficient just after completion of sleep return operation that returns to a job operable state from a sleep mode. Even in such as case, poor fixing occurs similarly as described before.

To overcome the problem, Japanese Unexamined Patent Publication JP-A 2004-126191 discloses an image forming apparatus in which storage of heat of a fixing roller is estimated and idle time of the fixing roller is changed after completion of warming-up according to the estimated storage of heat.

According to the image forming apparatus disclosed in JP-A 2004-126191, the period until completion of warming-up operation from power activation is measured, and storage of heat of the fixing roller is estimated based on the measurement result.

Japanese Unexamined Patent Publication JP-A 2006-119194 discloses an image forming apparatus in which time interval of conveying a recording medium to a fixing roller is prolonged or conveying speed of a recording medium is decreased. According to the image forming apparatus disclosed in JP-A 2006-119194, lowering of a surface temperature of a fixing roller is suppressed by reducing a quantity of heat absorbed in a recording medium from the fixing roller per unit time, thereby fixability can be improved.

In an image forming apparatus, when an external power source is a power source such as a private power generator or electric power is supplied to the image forming apparatus in a form of multiple connection (octopus wiring) together with other OA (Office Automation) equipment, power supply to the image forming apparatus becomes unstable state. Alternatively, impedance of a power supply line becomes large, resulting in the state that voltage varies by the variation of electric current. Furthermore, commercial alternating current source has relatively large variation to rated voltage, depending on power supply circumstance of each country.

When variation of voltage applied to an image forming apparatus is large and low voltage is applied to a heater provided inside a fixing roller, current conduction to the heater is insufficient, and quantity of heat supplied to a core metal of a fixing roller from the heater lacks.

For example, in the image forming apparatus disclosed in JP-A 2004-126192, when voltage applied is lower than rated voltage and current conduction to a heater is initiated, not in the state that applied voltage is lower than rated voltage and a surface temperature of a fixing roller is not sufficiently decreased to room temperature, but in the state that the fixing roller has a heated surface, because the surface temperature of the fixing roller is already high, the measurement time until completion of warming-up operation is short even though the applied voltage is lower than rated voltage. In this case, with respect to the image forming apparatus, storage of heat of the fixing roller is estimated using the measurement time until completion of the warming-up operation, and it is therefore judged that the fixing roller sufficiently stores heat. When job is started in this state, because voltage lower than rated voltage is applied to the heater, the heat storage of the fixing roller is not sufficient, and this may lead to occurrence of poor fixing.

In the image forming apparatus disclosed in JP-A 2006-119194, time interval of conveying a recording medium to a fixing nip region is prolonged or conveying speed of a recording medium is decreased. Therefore, printing speed is decreased, and high speed printing cannot be realized.

Summary of the invention

An object of the invention is to provide an image forming apparatus equipped with a fixing device of a heat fixing system, in which even though power supply is an unstable state, storage of heat of a fixing section can sufficiently be secured before initiation of an image forming operation, thereby occurrence of poor fixing can be prevented and additionally high speed printing can be realized.

The invention provides an image forming apparatus in which a toner image is formed by feeding a toner to an electrostatic latent image formed on an image bearing member by electrophotography, by a developing device, the apparatus comprising:

a transfer section which transfers the toner image borne on the image bearing member to a recording medium;

a fixing section which heats the image toner transferred to the recording medium to fix the image toner to the recording medium, the fixing section comprising a heating section having a heat source for generating heat by current conduction provided therein, and arranged so as to be rotation-driven around an axis thereof, a pressure section provided so as to form a pressure-contact part between the pressure section and the heating section by coming into pressure-contact with the heating section, and a driving section which rotates the heating section by current conduction;

a power source which applies a voltage to the fixing section;

a voltage detection section which detects a voltage applied to the fixing section from the power source;

a temperature detection section which detects a temperature of a surface of the heating section; and

a fixing control section which controls the operation of the fixing section, the fixing control section comprising: a switching control section which controls switching on and off conduction operations between the heat source and the power source, and between the driving section and the power source; a voltage judgment section which judges as to whether or not a detected voltage by the voltage detection section is a given threshold voltage or more; a temperature judgment section which judges as to whether or not a detected temperature by the temperature detection section is a given preset temperature or higher; and a preparation control section which controls a preparation operation of the heating section before performing the operation of forming a toner image on the image carrier, wherein when the switching control section switches a conduction state to the heat source and the driving section from an off conduction state to an on conduction state and the voltage judgment section judges that the detected voltage by the voltage detection section is less than the threshold voltage, the preparation control section controls the preparation operation of the heating section such that the switching control section continues the on conduction state over a given idle time after the temperature judgment section has judged that the detected temperature by the temperature detection section is a preset temperature or higher.

According to the invention, the image forming apparatus comprises the transfer section, the fixing section, the power source, the voltage detection section, the temperature detection section and the fixing control section. The fixing control section comprises the switching control section, the voltage judgment section, the temperature judgment section and the preparation control section. The preparation control section controls the preparation operation of the heating section before performing the operation of forming a toner image of an image carrier. When the switching control section switches a conduction state to the heat source and the driving section from an off conduction state to an on conduction state and the voltage judgment section judges that the detected voltage by the voltage detection section is less than the threshold voltage, the preparation control section controls the preparation operation of the heating section such that the switching control section continues the on conduction state to the heat source and the driving section over a given idle time after the temperature judgment section has judged that the detected temperature by the temperature detection section is a given preset temperature or higher.

When the voltage judgment section judges that the detected voltage by the voltage detection section is less than the threshold voltage, the preparation control section controls the preparation operation of the heating section such that the switching control section continues the on conduction state to the heat source and the driving section over a given idle time. Therefore, even though voltage applied to the fixing section is low and current conduction to the heating section is insufficient, storage of heat of the heating section can sufficiently be secured before performing image forming operation by an image creating section, that is, before initiation of image forming operation, and as a result, occurrence of poor fixing can be prevented. Furthermore, because storage of heat of the heating section is sufficiently secured, even though a large amount of a recording medium is continuously conveyed to a pressure-contact part at the time of fixing and heat of the heating section is absorbed by the recording medium, lowering of the temperature of the heating section can be suppressed by utilizing the heat storage. As a result, time interval that the recording medium is conveyed to the pressure-contact part can be shortened or conveying speed of the recording medium can be increased, thereby high speed printing can be realized.

Furthermore, it is preferable that when voltage is applied to the fixing section from the power source or when return operation from a sleep mode is initiated, the preparation control section controls the preparation operation of the heating section.

According to the invention, when voltage is applied to the fixing section from the power source or when return operation from a sleep mode is initiated, the preparation control section controls the preparation operation of the heating section. Just after voltage has been applied to the fixing section from the power source or just after the return operation from a sleep mode has been initiated, the surface temperature of the heating section is lower than the fixable temperature. This requires that the temperature of the heating section must be higher than the preset temperature before performing the image forming operation. In such a case, when low voltage is applied to the fixing section, current conduction to the heat source becomes insufficient, and quantity of heat supplied to the heating section from the heat source is insufficient.

When voltage is applied to the fixing section from the power source or when return operation from a sleep mode is initiated, the preparation control section controls the preparation operation of the heating section. This control makes it possible to sufficiently secure storage of heat of the heating section before initiating the image forming operation, can prevent occurrence of poor fixing, and additionally can realize high speed printing.

Furthermore, it is preferable that the image forming apparatus comprises a memory section which memorizes a first table linking the idle time and a voltage value, and

the preparation control section determines the idle time using the first table.

According to the invention, the image forming apparatus comprises a memory section which memorizes a first table linking the idle time and a voltage value, and the preparation control section determines the idle time using the first table. By this embodiment, the preparation control section can determine the idle time according to the voltage applied to the fixing section.

Furthermore, it is preferable that the image forming apparatus comprises a reserved job number detection section which detects a number of reserved jobs corresponding to a reserved number of sheets of recording mediums on which an image is formed,

the memory section memorizes a second table linking the idle time, the number of reserved jobs and a voltage value, and

the preparation control section determines the idle time using the second table.

According to the invention, the image forming apparatus comprises the reserved job number detection section which detects the number of reserved jobs corresponding to the reserved number of sheets of recording mediums on which an image is formed. The memory section memorizes a second table linking the idle time, the number of reserved jobs and a voltage value, and the preparation control section determines the idle time using the second table. By this embodiment, the preparation control section can determine the idle time in response to the voltage applied to the fixing section from the powder source and the number of reserved jobs.

Brief description of the drawings

Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:

FIG. 1 is a view schematically showing the constitution of an image forming apparatus according to one embodiment of the invention;

FIG. 2 is a view schematically showing the constitution of an image forming unit;

FIG. 3 is a cross-sectional view showing the constitution of a fixing section;

FIG. 4 is a block diagram showing the constitution of a control section of the image forming apparatus;

FIG. 5 is a block diagram showing the constitution of a fixing section control circuit;

FIGS. 6A and 6B are flow charts showing the operation of the image forming apparatus;

FIG. 7 is a timing chart showing timing of the operation in the image forming apparatus;

FIGS. 8A and 8B are flow charts showing the operation of the image forming apparatus; and

FIG. 9 is a timing chart showing the timing of operation in the image forming apparatus.

Detailed description

Now referring to the drawings, preferred embodiments of the invention are described below.

FIG. 1 is a view schematically showing the constitution of an image forming apparatus 1 according to one embodiment of the invention. The image forming apparatus 1 comprises an image forming section 2, an intermediate transfer section 3, a secondary transfer section 4, a recording medium feeding section 5, and a fixing section 6. The image forming apparatus 1 further comprises a display section, an operation section and a control section, which are not shown in FIG. 1.

(Image Forming Section)

The image forming section 2 which is an image creating section comprises image forming units 10y, 10m, 10c and 10b. The image forming units 10y, 10m, 10c and 10b form an electrostatic latent image corresponding to a digital signal of each color (hereinafter referred to as "image information"), develop the electrostatic latent image, and form a toner image by a toner of each color. Specifically, the image forming unit 10y forms a toner image corresponding to the image information of yellow color, the image forming unit 10m forms a toner image corresponding to the image information of magenta color, the image forming unit 10c forms a toner image corresponding to the image information of cyan color, and the image forming unit 10b forms a toner image corresponding to the image information of black color.

As the image forming units 10y, 10m, 10c, and 10b are of the same configuration except that a yellow developer, magenta developer, cyan developer, and black developer are used in the respective image forming units, and that, among pieces of image information inputted into the image forming section 2, a pixel signal corresponding to a yellow component image, a pixel signal corresponding to a magenta component image, a pixel signal corresponding to a cyan component image, and a pixel signal corresponding to a black component image are inputted into the respective image forming units, hereafter, the image forming unit 10y for yellow will be shown as a representative example, and a description of the others will be omitted.

The image forming units 10 and the like for the individual colors, when separately shown, will be represented with alphabetical suffixes: y (yellow), m (magenta), c (cyan), and b (black), affixed thereto. The image forming units 10y, 10m, 10c, and 10b are arranged side by side in the order named, from an upstream side to a downstream side, in a direction of movement (a sub-scanning direction) of an intermediate transfer belt 21, that is, in the direction of an arrow 27.

FIG. 2 is a view schematically showing the constitution of the image forming unit 10y. The image forming unit 10y comprises a photoreceptor drum 11y, a charging roller 12y, a light scanning unit 13y, a developing device 14y and a drum cleaner 15y.

The photoreceptor drum 11y is an image bearing member on the surface of which a toner image of yellow color is to be formed, is supported so as to be rotation-driven around an axis thereof, and comprises cylindrical, columnar or thin film sheet-shaped (preferably, cylindrical) conductive substrate and a photosensitive layer formed on the surface of the conductive layer, which are not shown.

The photoreceptor drum 11y can use any photoreceptor drums conventionally used in this field, and can use, for example, a photoreceptor drum connected to GND (ground) potential, comprising an aluminum pipe stock as a conductive substrate and an organic photosensitive layer which is a photosensitive layer formed on the surface of the aluminum pipe stock.

The organic photosensitive layer may be one formed laminating a charge generating layer including a charge generating substance, and a charge transporting layer including a charge transporting substance, or may be one having the charge generating substance and charge transporting substance included in one layer. The thickness of the organic photosensitive layer is not particularly limited but is, for example, 20 .mu.m. Also, an undercoat layer between the organic layer and conductive substrate may be provided. Furthermore, a protecting layer may be provided on the surface of the organic photosensitive layer.

The photoreceptor drum 11y rotates at a peripheral speed of, for example, 220 mm/sec in a counterclockwise direction to the paper of FIG. 2 by a driving section which is not shown in FIG. 2. The driving section of the photoreceptor drum 11y is controlled by an image forming section control section described hereinafter, and rotation speed of the photoreceptor drum 11y is controlled by the image forming section control section.

The charging roller 12y is a charging section which charges the surface of the photoreceptor drum 11y with potential of predetermined polarity. As the charging section, it is not limited to only the charging roller 12y and the charging roller 12y can be replaced by a brush-type charging device, a charger-type charging device, or a corona charging device such as a scorotron charger.

The light scanning unit 13y is a latent image forming section which irradiates the charged surface of the photoreceptor drum 11y with a laser beam corresponding to the yellow image information, and forms an electrostatic latent image corresponding to the yellow image information on the surface of the photoreceptor drum 11y. A semiconductor laser element or the like is used as a laser beam light source.

The developing device 14y is provided facing the photoreceptor drum 11y, and is a developing section in which of a yellow toner and a carrier that are contained in a two-component developer 16y, the yellow toner is borne on a surface of a developing sleeve 17y, the toner is regulated to a thickness of a given amount by a layer thickness regulating member 18y and conveyed to the surface of the photoreceptor drum 11y, and an electrostatic latent image formed on the surface of the photoreceptor drum 11y is developed to visualize the latent image. A developer can use a one-component developer free of a carrier.

The developing sleeve 17y is rotation-driven in the same direction as a direction of rotational drive of the photoreceptor drum 11y, in a developing nip region which is close to the photoreceptor drum 11y.

The drum cleaner 15y has the function that after a toner image of yellow color on the surface of the photoreceptor drum 11y is intermediately transferred to an intermediate transfer belt 21, the residual yellow toner on the surface of the photoreceptor drum 11y, which is not intermediately transferred to the intermediate transfer belt 21 is removed and collected.

According to the image forming unit 10y, for example, 1200 V is applied to the charging roller 12y by a power source (not shown) while rotationally driving the photoreceptor drum 11y around its axis, and the photoreceptor drum 11y is discharged, thereby charging the surface of the photoreceptor drum 11y to, for example, 600 V. Next, the charged surface of the photoreceptor drum 11y is irradiated with the laser beam corresponding to the yellow image information from the light scanning unit 13y, forming an electrostatic latent image with an exposure potential of -70 V corresponding to the yellow image information.

The surface of the photoreceptor drum 11y and the yellow toner borne on the surface of the development sleeve 17 are close to each other. Direct current voltage of -450V is applied as development potential to the developing sleeve 17y, the yellow toner is adhered to the electrostatic latent image by voltage difference between the developing sleeve 17y and the photoreceptor drum 11y, and an yellow toner image is formed on the surface of the photoreceptor drum 11y. The yellow toner image is intermediately transferred to the intermediate transfer belt 21 which comes into pressure-contact with the surface of the photoreceptor drum 11y and is driven in a direction of an arrow 27, as described hereinafter. The residual yellow toner on the surface of the photoreceptor drum 11y is removed and collected by the drum cleaner 15y. Formation operation of the yellow toner image is repeatedly performed in the same manner as above.

Two-component developers 16y, 16m, 16c and 16b used in the image forming apparatus 1 of the present embodiment are described in detail below. The two-component developers 16y, 16m, 16c and 16b comprise a toner and a carrier.

The toner comprises toner particles each containing a binder resin, a colorant, and a release agent. As the binder resin, ingredients customarily used in this field can be used, and examples thereof include polystyrene, a homopolymer of styrene substitute, a styrene-type copolymer, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, polyester, and polyurethane. The binder resins may be used each alone, or two or more thereof may be used in combination.

Among the above kinds of binder resin, for the color toner, preferable is the binder resin which has a softening temperature of 100.degree. C. to 150.degree. C. and a glass transition temperature of 50.degree. C. to 80.degree. C., and particularly preferable is polyester which has a softening temperature and a glass transition temperature in the above ranges, from the aspect of storage stability, durability, etc. Polyester in a softened or fused state is high in transparency. In the case where polyester is used as the binder resin, when a multicolor toner image composed of overlaid toner images of yellow, magenta, cyan, and black, is fixed on a recording sheet 8 by the fixing section 6 described hereinafter, the polyester itself becomes transparent, leading to sufficient color development by subtractive color mixture.

As the colorant, it is possible to use pigments and dyes for toner which have been conventionally used in the electrophotographic image forming technique. Examples of the pigment include an organic pigment such as azo pigment, benzimidazolone pigment, quinacridone pigment, phthalocyanine pigment, isoindolinone pigment, isoindoline pigment, dioxazine pigment, anthraquinone pigment, perylene pigment, perynone pigment, thioindigo pigment, quinophthalone pigment, or metal complex pigment; an inorganic pigment such as carbon black, titanium oxide, molybdenum red, chrome yellow, titanium yellow, chrome oxide, or Berlin blue; and metal powder such as aluminum powder. The pigments may be used each alone, or two or more thereof may be used in combination.

As the release agent, wax can be used, for example. It is possible to use the wax which is customarily used in this field, and examples thereof include polyethylene wax, polypropylene wax, and paraffin wax.

The toner may contain, other than the binder resin, colorant, and release agent, one or two or more additives for general use in toner, such as a charge control agent, a fluidity improving agent, a fixing promoting agent, and a conductive agent.

The toner can be produced by the conventional methods such as a pulverization method, a suspension polymerization method or an emulsion coagulation method. The pulverization method obtains a toner by melt-kneading a colorant, a release agent and the like with a binder resin, and pulverizing the melt-kneaded product. The suspension polymerization method obtains a toner by uniformly dispersing monomers such as a binder resin, a colorant and a release agent, and polymerizing those monomers. The emulsion coagulation method obtains a toner by coagulating a binder resin, a colorant and a release agent by a coagulating agent, and heating fine particles of the aggregates obtained.

A volume average particle size of the toner is not particularly limited, but is preferably from 2 .mu.m to 7 .mu.m. When the volume average particle size of the toner is appropriately small as above, coverage of the toner to a recording medium 8 is increased. This makes it possible to achieve high image quality with a small amount of the toner adhered and reduction in the amount of the toner consumed.

When the volume average particle size of the toner is less than 2 .mu.m, fluidity of the toner is decreased, and feeding, stirring and charging of the toner become insufficient during the development operation. This leads to insufficient amount of a toner fed to the photoreceptor drum 11 and increase in amount of a toner having reverse polarity, and a high quality image may not be obtained. When the volume average particle size of the toner exceeds 7 .mu.m, toner particles having a large particle size which are difficult to be softened up to the central portion at the time of fixing are increased. As a result, fixability of a toner image to the recording medium 8 is deteriorated, and additionally, coloration of an image is deteriorated. Particularly, in the case of fixing to an OHP sheet, an image becomes murky.

The toner used in the image forming apparatus 1 of the present embodiment is a negatively charging insulating non-magnetic toner having a glass transition temperature of 60.degree. C., a softening temperature of 120.degree. C., and a volume average particle size of 6 .mu.m. To obtain an image density having a reflection density measurement value by 310 manufactured by X-Rite of 1.4 using this toner, the toner is required in an amount of 5 g/m.sup.2 on the surface of the recording medium 8.

The toner comprises a polyester having a glass transition temperature of 60.degree. C. and a softening temperature of 120.degree. C. as a binder resin, a pigment of each color as a colorant in an amount of 12% by weight based on the total amount of the toner, and a low molecular polyethylene wax having a glass transition point of 50.degree. C. and a softening point of 70.degree. C. as a release agent in an amount of 7% by weight based on the total amount of the toner. The low molecular polyethylene wax used as a release agent in this toner is a wax having a glass transition temperature and a softening temperature lower than those of the polyester used as a binder resin.

The carrier can use magnetic particles. Examples of the magnetic particles include metals such as iron, ferrite and magnetite, and alloys of those metals and metals such as aluminum and lead. Of those, ferrite is preferred.

A resin-coated carrier comprising magnetic particles having a resin applied on the surface thereof, or a resin-dispersed carrier comprising a resin and magnetic particles dispersed therein may be used as the carrier. Resins for covering the magnetic particles are not particularly limited. Examples of the resin include an olefinic resin, a styrenic resin, a styrene-acryl resin, a silicone resin, an ester resin and a fluorine-containing polymer resin. The resin used in the resin-dispersed carrier is not particularly limited. Examples the resin include a styrene-acryl resin, a polyester resin, a fluorine resin and a phenolic resin.

A volume average particle size of the carrier is not particularly limited. Considering high image quality, the volume average particle size is preferably from 30 .mu.m to 50 .mu.m. The resistivity of the carrier is preferably 10.sup.8 .OMEGA.cm or more, and more preferably 10.sup.12 .OMEGA.cm or more.

The volume resistivity of the carrier is a value obtained from a current value determined as follows. The carrier particles are put into a container having a cross-sectional area of 0.50 cm.sup.2, and then tapped. Subsequently, a load of 1 kg/cm.sup.2 is applied by use of a weight to the particles which are held in the container. When an electric field of 1000 V/cm is generated between the load weight and a bottom electrode of the container by application of voltage, a current value is read. When the resistivity of the carrier is low, an electric charge will be injected into the carrier upon application of bias voltage to a developing sleeve 17y, thus causing the carrier particles to be more easily attached to the photoreceptor drum 11y. Further, breakdown of the bias voltage is more liable to occur.

The magnetization intensity (maximum magnetization) of the carrier is preferably 10 emu/g to 60 emu/g, and more preferably, 15 emu/g to 40 emu/g. Although the magnetization intensity depends on the magnetic flux density of the developing sleeve 17y, under conditions of the general magnetic flux density of the developing sleeve 17y, there is a fear that no magnetic constraint force acts in the event of less than 10 emu/g, causing a carrier dispersion. Also, when the magnetization intensity exceeds 60 emu/g, with a non-contact development in which a magnetic brush of the carrier is too high, it is difficult to keep a non-contact condition with the photoreceptor drum 11y. Also, with a contact development, there is a fear that a brush mark is likely to appear in the toner image.

It is preferable that the shape of the carrier is a spherical or flat shape.

A blend ratio of toner and carrier in the developers 16y, 16m, 16c, and 16b is not particularly limited, and it is sufficient to appropriately select it depending on the type of toner and carrier.

(Intermediate Transfer Section)

As shown in FIG. 1, the intermediate transfer section 3 comprises an intermediate transfer belt 21, intermediate transfer rollers 22y, 22m, 22c and 22b, supporting rollers 23 and 25, and a belt cleaner 26. In the present embodiment, the transfer section is constituted of the intermediate transfer section 3 and a secondary transfer section 4 described hereinafter.

The intermediate transfer belt 21 is an endless belt-shaped image bearing member which is supported around the supporting rollers 23 and 25 and a supporting roller 24 described hereinafter with tension and forms a loop-shaped movement passage, and is rotation-driven in a direction of the arrow 27, that is, such that an image bearing surface facing the photoreceptor drums 11y, 11m, 11c and 11b moves toward the photoreceptor drum 11b from the photoreceptor 11y, in substantially the same peripheral speed as the photoreceptor drums 11y, 11m, 11c and 11b.

As the intermediate transfer belt 21, it is possible to use, for example, a polyimide film having thickness of 100 .mu.m. As a material of the intermediate transfer belt 21, it not being limited to polyimide, it is possible to use a film configured of a synthetic resin, such as polycarbonate, polyamide, polyester, or polypropylene, various kinds of rubber, or the like.

The film made of a synthetic resin or any kind of rubber contains an electrically conductive material, such as furnace black, thermal black, channel black, or graphite carbon, in order to adjust an electric resistance value with which it acts as the intermediate transfer belt 21. Also, a covering layer configured of a fluorine resin composition, fluorine-containing rubber, or the like, which has a low adhesion to toner, may be provided on the intermediate transfer belt 21. As a component material of the covering layer, examples thereof include polytetrafluoroethylene (PTFE) and PFA (a copolymer of PTFE and perfluoroalkyl vinyl ether). The covering layer may contain an electrically conductive material.

An image bearing surface of the intermediate transfer belt 21 comes into pressure-contact with the photoreceptor drums 11y, 11m, 11c, and 11b in the order just stated from the upstream side in the rotational direction of the intermediate transfer belt 21. Positions where the intermediate transfer belt 21 comes into pressure-contact with the photoreceptor drums 11y, 11m, 11c, and 11b, are positions where toner images of respective colors are transferred.

The intermediate transfer rollers 22y, 22m, 22c, and 22b are roller members which are respectively opposed to the photoreceptor drums 11y, 11m, 11c, and 11b with the intermediate transfer belt 21 interposed therebetween and come into pressure-contact with a reverse side of the image bearing surface 21a of the intermediate transfer belt 21 and which are disposed so as to be rotation-driven about respective axes of the rollers by a driving section (not shown).

For each of the intermediate transfer rollers 22y, 22m, 22c, and 22b, a roller member is used, for example, which is composed of a metal shaft and a conductive layer covering a surface of the metal shaft.

The metal shaft is, for example, formed of a metal such as stainless steel. A diameter of the metal shaft is not particularly limited, and preferably from 8 mm to 10 mm.

The conductive layer is formed of a conductive elastic body or the like material. As the conductive elastic body, a material customarily used in this field is applicable, and examples thereof include ethylene-propylene rubber (hereinafter described as EPDM), foamed EPDM, and urethane foam, which contain a conductive material such as carbon black. Owing to the conductive layer, high voltage is evenly applied to the intermediate transfer belt 21.

Owing to the conductive layer, high voltage is evenly applied to the intermediate transfer belt 21.

Since the toner images formed on the surfaces of the photoreceptor drum 11y, 11m, 11c, and 11b are transferred onto the intermediate transfer belt 21, intermediate transfer bias voltage is applied to the intermediate transfer rollers 22y, 22m, 22c, and 22b through a constant voltage control, which bias has a polarity reverse to that of the polarity of the charged toner. By so doing, the toner images of yellow, magenta, cyan, and black formed on the photoreceptor drums 11y, 11m, 11c, and 11b are sequentially transferred and overlaid on top of one another on the image bearing surface of the intermediate transfer belt 21, thus forming a multicolor toner image. Note that in the case where image information of only part of yellow, magenta, cyan, and black is inputted, a toner image is formed by only an image forming unit 10 corresponding to a color of inputted image information, among the image forming units 10y, 10m, 10c, and 10b.

Among the supporting rollers 23, 24, and 25, the supporting rollers 23, and 25 are disposed so as to be rotation-driven about respective axes thereof by a driving section (not shown), support the intermediate transfer belt 21 therearound with tension and rotated in the direction of the arrow 27 by the supporting rollers 23, 24, and 25. For each of the supporting rollers 23, 24, and 25, an aluminum-made cylinder (a pipe-shaped roller) is used, for example, having a diameter of 30 mm and a thickness of 1 mm. The supporting roller 24 comes into pressure-contact with a later-described secondary transfer roller 28 with the intermediate transfer belt 21 interposed therebetween, thus forming a secondary transfer nip region, and is electrically grounded.

The description continues in the full USPTO document.